1. Field of the Invention
The present invention relates to a circuit for detecting a signal component of a periodic signal which includes a relatively large noise component.
2. Description of Related Art
Heretofore, this type of signal detecting circuit has typically been composed of a peak hold circuit receiving an input signal, and a voltage comparator having two inputs, one of which receives the input signal and the other of which is connected to the output of the peak hold circuit.
However, the peak hold circuit ordinarily requires a charge storing capacitor which must be of substantial capacitance to hold the potential, and which therefore is very difficult to provide in integrated circuit form. Accordingly, when the above type of signal detecting circuit is constructed in the form of an integrated circuit, the charge storing capacitor is externally connected to the integrated circuit, and so, the signal detecting integrated circuit has to have not only signal input and output terminals but also an external terminal for connection of the charge storing capacitor.
Furthermore, in the case of no signal, the output signal level of the peak hold circuit becomes substantially the same as that at the signal input terminal. In other words, the two inputs of the voltage comparator become substantially equal. On the other hand, since noises themselves vary in amplitude or level, in a no signal condition, noises will often erroneously be detected as signal components by the comparator.
In addition, the voltage comparator will properly operate when the input signals applied to the comparator are within the common-mode input voltage range. However, in the above signal detecting circuit, since the reference input voltage of the voltage comparator follows the peak level of the input signal, the amplitude of the input signal must be limited to be within the common-mode input voltage range for proper operation, and therefore, the circuit will often malfunction when too large of a signal is applied.